“Pop-up mass transit” that can be directed by the riders themselves. Services that not only find you a parking space but valet your car — wherever you are. Smart energy solutions that control heating and cooling based on when you are in the room or turn on the sprinklers only if Mother Nature isn’t cooperating.

These are examples of “urban tech” solutions aimed at simultaneously making it easier to live in cities and helping to reduce the carbon footprint. They join other solutions that have grown in popularity in the past couple of years: ride-, bike- and room-sharing, delivery services, and crowd-sourced information portals — all of which can be easily accessed with your smartphone.

The big idea: Reimagine the way things have been done for decades and come up with ways to make urban living more efficient, more convenient, and often cleaner and greener. Increasingly, it’s the small, nimble startups that hop on the opportunity.

“It is about the opportunity,” said Stonly Baptiste, co-founder of Urban.Us, a Miami startup born to serve this trend. “There are always going to be problems in cities, and we are going to continue to see interesting startups that are solving these problems.”

Baptiste, who has co-founded or helped run a number of tech companies, and fellow Urban.Us co-founder Shaun Abrahamson, an investor in early-stage companies, are among the early adopters trying to power the success of these startups. Urban.Us, a public benefit corporation that funds startups that help cities, has been growing a network of 300 advisors to help those startups and to identify new companies and trends. Urban.Us also convenes an annual conference in April, bringing to Wynwood some of the best urban tech entrepreneurs from around the world.

Some of these urban tech startups use the “sharing economy,” which allows people to take advantage of underutilized assets, like a spare bedroom or time in their schedule to drive. Uber and Lyft, both ride services that entered South Florida — in violation of local regulations — this summer, are perhaps the most visible of this new breed of technology and in hundreds of cities around the globe.

Other urban tech startups, such as Nest, are based on technology helping you to make your spaces — home, work, car — safer, as well as cleaner and greener. Still others, like Waze for traffic, use the crowd or social media to help you navigate the urban jungle.

Fueling this trend is this global urban reality: More than half the world now lives in cities, and in 30 years, 70 percent will live in cities, Abrahamson said. Large cities now consume two-thirds of the world’s energy and create over 70 percent of global CO2 emissions. In the United States, 80 percent of residents already live in cities, stretching resources. And with urban renaissances taking place in the heart of many cities, downtown neighborhoods are experiencing unprecedented growth.

You don’t have to look far to see this trend playing out: The population of downtown Miami — Brickell through Wynwood — has doubled since 2000, to a projected 80,750 residents in 2014. Forty-six percent of those residents are young professionals, ages 25 to 44, according to the Miami Downtown Development Authority’s just-released 2014 Demographics Report. During the day, 222,000 people inhabit Miami’s urban core.

And the growth is not expected to let up. The Miami DDA report projects a population of nearly 100,000 in five years. Fueling that projection: There are about 23,000 condo units currently in the pipeline in the greater downtown region, according to the DDA. Since 2000, more than 22,000 condo units have come online in downtown Miami.

Fort Lauderdale is also experiencing an urban living surge, with 23 condo towers now in some stage of development, according to CraneSpotter’s research. Locally and around the globe, cities struggle with how to handle the surge — from infrastructure to energy to transit to safety. Some governments are actively involved in addressing these issues. But increasingly, it is private businesses, particularly startups, that are creating solutions to ease conditions or potentially change habits.

“If cities don’t change dramatically in the next 30 years, then we have to rethink society. Mayors and people in the cities are ultimately going to decide what will happen with climate change. Who solves this? I don’t think it is fair to ask local governments to do this,” said Abrahamson, in an interview in Urban.Us’ Wynwood office.

Many companies and organizations are stepping up. Urban.Us will hold its second Smart City Startups conference April 23-24, which will bring together and showcase 100 top startups from around the world solving city problems and explore issues ranging from funding to partnerships. It’s just one of several initiatives going on:

The John S. and James L. Knight Foundation launched a Knight Cities Challenge in the 26 cities it supports, including Miami. Funded with $5million per year for three years, the program seeks out and will fund the best ideas “to make cities more successful.” The call for entries is open until Nov.14.

Code for America is a national organization that pairs technologists with governments to help find urban solutions, and in September it announced Miami-Dade County was selected for a fellowship. Three technology fellows will work with the county government for a year on some of its biggest challenges. South Florida also has active Code for Miami and Code for Fort Lauderdale brigades.

The global nonprofit Ashoka Foundation supports social entrepreneurs, some of whom are focused on solving urban challenges, and Ashoka has an active chapter here. Besides seeking out social entrepreneurs within Miami, Ashoka South Florida has also been working to bring some of the global network’s solutions to the city, said Maria Escorcia, Ashoka South Florida’s director.

Sascha Haselmayer, an Ashoka fellow in Barcelona, runs Citymart.org. Citymart aims to revolutionize the procurement process by working with cities worldwide to come up with solutions for their problems through open challenges. For instance, San Francisco is running a challenge to find ways to eliminate traffic-related deaths. About 41 solutions were presented, and there is now a selection process under way.

Moscow sought solutions for noise pollution through a Citymart challenge. About 65 percent of its residents, or 8million people, are literally kept up at night by the noise levels in their apartments. One company proposed noise barriers built out of solar panels so that they do double duty. Another company proposed a device that sticks to a window and vibrates the glass so it becomes an inverse loudspeaker and cancels out noise outside. “It’s one of those things that is still in the prototype stage, but you can imagine if it can help 8million people in Moscow, the number of people you could help worldwide would be amazing,” Haselmayer said, in a phone interview from Spain.

While the solutions can come from anywhere — big companies, social services, small businesses, etc. — Haselmayer said 98 percent have been coming from startups.

“Solutions come in all shapes and sizes; very few of them require you to make a multibillion-dollar, 25-year investment to address the problem,” Haselmayer said. “Citizens increasingly are part of discovering these solutions. Ten years ago, it would have been unthinkable that citizens would have become part of designing their own taxi system like Uber.

“For the problems cities gave us over the last year, we found 10,000 solutions they had never seen before,” Haselmayer said. “Startup are better at understanding the niche product. Any big company could engineer it also, but it is the startup that makes it its only business model. … Entrepreneurs put everything on the line to solve your problem.”

Citymart recently set up a U.S. base in New York, and Haselmayer and Ashoka South Florida would like to bring Citymart here. Haselmayer is also an advisor for Urban.Us, was a speaker at last year’s Smart City Startups conference and is working with Urban.Us on next year’s event.

For its part, Urban.Us has looked at more than 200 companies in the past year. Abrahamson and Baptiste say they look for companies that help solve core city problems, such as water, waste, energy, transport and safety.

“For most of us, these are the things we don’t notice unless they stop working. But these are also the things that require new approaches as cities confront the triple threat of climate change, increased populations and constrained budgets,” Abrahamson said.

The Urban.Us founders choose companies with strong teams and solutions that they believe can scale to 100 cities within five years. The 14 startups Urban.Us is funding so far run the gamut from one called Skycatch that uses airborne drones to take images of infrastructure to HandUp, a tech-enabled solution to directly help the homeless and combat poverty.

Urban.Us invests in personal mobility startups such as BikeSpike, an OnStar for cyclists, and the very portable Onewheel, a single-wheel electrified self-balancing skateboard-like product. “It kind of looks like a toy, but it can compete with biking but be more convenient and fun,” said Abrahamson, who has led product development and digital strategy teams. “One of the challenges with mass transit is getting to the station or getting from the station to work.”

Not all are driving alternatives. Dash is an iPhone and Android mobile app that helps you save money and improves your driving experience by telling you what’s going on with your car and your driving. Dash already has 130,000 users, 70 percent of them international and 30 percent domestic. Dash is on track for 150,000 to 165,000 by year’s end.

“We’ve had well over 10million miles driven in over 1million trips in 150 countries,” CEO Jamyn Edis said. Florida is the company’s fourth-biggest market.

Dash consists of a low-cost device that when used with a smartphone gives virtually any car of 1996 vintage or younger a brain and a voice, he said, allowing the driver to drive smarter, safer and more fuel-efficiently. For instance, the “check engine light” may or may not indicate a serious problem: Dash will tell you the underlying cause. What sets it apart from competitors is that Dash analyzes 300 sources of data. And beyond being utilitarian, Dash can make driving more like a game, Edis said.

ValetAnywhere, based in New York, will meet you wherever you are and park your car for you. Meanwhile, a locally based startup, ParkJockey, is getting ready to launch PlumValet, which will also offer this service, including in Miami.

Miami-based WhatUpBridge, an Urban.Us very early-stage investment, seeks to track local infrastructure and notify commuters of delays. In a beta test, WhatUpBridge is monitoring the Brickell Avenue Bridge in downtown Miami, allowing beta users to receive notifications about the status of the bridge based on their schedule, said Allyn Alford, founder of WhatUpBridge.

Business-to-government startups also have their place in urban tech. SeamlessDocs, born in Fort Lauderdale but now New York-based, makes pdfs and government forms “smart” — simple and trackable for both govenment workers and residents who want to avoid sending (and resending and resending) forms with no sense of who sees them and when. The company is already working with Los Angeles, Jersey City, N.J., and San Juan, Texas, and can save budget strapped cities significant dollars. “We saved the city of Los Angeles $40,000 a year by automating one single form,” said CEO Jonathon Ende.

Urban.Us helped SeamlessDocs find its market niche, Ende said, adding that SeamlessDocs’ chief technology officer is based in South Florida, it’s opening a Miami office and is aggressively hiring. “Urban.Us is creating a knowledge network and support system — I call Shaun the shepherd,” Ende said.

Rach.io, a device sold online and in Home Depot and Apple stores, adjusts irrigation systems as the weather changes and can save a residence or business 20 percent to 30 percent off its water bill. Rach.io is already sold across the country and South Florida is one of its biggest markets, said CEO Christopher Klein. At an earlier stage of development, Flair aims to save energy by making your air vents “smart.”

Abrahamson and Baptiste say that real estate is becoming a stakeholder in these startups. “They look at something like Skycatch that can save them an hour a day in inspections, or Rach.io that can save them 30 percent water, and that’s meaningful,” Abrahamson said. Real estate developers can also help startups pilot-test their concepts and be potential investors, he added, and the younger generation of multi-generation family real-estate businesses often is interested in technology innovation.

Increasingly, according to Abrahamson, solutions solving some of the world’s biggest urban challenges will be delivered not by government, but distributed in Home Depot or Apple stores — and coming soon to a condo near you.